1. Field of the Invention
The present invention relates to a fiber optic amplifier for amplifying signal light including one or more wavelengths within a predetermined wavelength band.
2. Related Background Art
Demands for optical amplifiers have rapidly been enhancing in order to respond to needs for increasing communications capacity, elongating repeater distances, and the like in optical communications. Among others, fiber optic amplifiers have a great merit from the viewpoint of reducing cost and enhancing functions in optical communications systems, since they can amplify signal light including one or more wavelengths within a predetermined wavelength band without converting them into electricity.
In general, a fiber optic amplifier comprises an optical fiber for amplification, doped with a rare-earth element, and a pumping light supply system for supplying pumping light to the optical fiber for amplification. The pumping light supply system usually includes a semiconductor laser, which is a pumping light source, and an optical coupler for guiding the pumping light into the optical fiber for amplification. In such a configuration, signal light is amplified within the optical fiber for amplification due to the stimulated emission light emitted from the rare-earth element excited by the pumping light. For example, the erbium-doped fiber optic amplifier whose optical fiber for amplification is doped with erbium, as a rare-earth element, collectively amplifies the signal light in a 1.55 xcexcm wavelength band when pumping light having a wavelength of 1.48 xcexcm or 0.98 xcexcm is supplied thereto.
The inventors have studied the prior art mentioned above and, as a result, found problems as follows. Namely, it has been known that, when fiber optic amplifiers are utilized in an environment in which they are inevitably exposed to radiation, e.g., in the vicinity of an atomic furnace, in outer space, or the like, glass defects occur upon exposure to the radiation (see Chie Fukuda et,al., xe2x80x9cHydrogen and radiation resistance of erbium-doped fibers,xe2x80x9d OFC""94 Technical Digest, pp.304-305, FF3). The occurrence of such glass defects would increase transmission loss of the signal light or pumping light, thereby deteriorating signal amplification characteristics of the fiber optic amplifiers. FIG. 1 is a graph showing a measured relationship between radiation dose (rad) and transmission loss (dB/m) with respect to light having a wavelength of 1.3 xcexcm concerning an erbium-doped optical fiber (optical fiber for amplification). The erbium-doped optical fiber used for this measurement has a relative refractive index difference of 2% with respect to a cladding, and an outside diameter of 3 xcexcm, while being co-doped with 1.0% by weight of aluminum and 0.1% by weight of erbium. From FIG. 1, it can be seen that the transmission loss of the optical fiber for amplification installed in a radiation environment increases as the radiation dose enhances. On the other hand, FIG. 2 is a graph showing a relationship between radiation dose and gain concerning a counter-propagating pumping type optical fiber amplifier equipped with an erbium-doped optical fiber. Here, the wavelength of pumping light is 1.48 xcexcm, the length of the erbium-doped optical fiber is 30 xcexcm, the wavelength of signal light is 1.55 xcexcm, and the input power of signal light is 35 mW. From FIG. 2, it can be seen that the gain of the optical fiber decreases as the radiation dose increases.
In order to overcome problems such as those mentioned above, it is an object of the present invention to provide a fiber optic amplifier comprising a structure capable of maintaining favorable signal amplification characteristics even in an environment exposed to radiation.
The fiber optic amplifier according to the present invention is an optical apparatus for amplifying signal light including one or more wavelengths within a predetermined wavelength band, and comprises, at least, an optical fiber for amplification having an entrance end for taking in signal light from a transmission line and an exit end for sending out amplified signal light onto the transmission line, the optical fiber for amplification constituting a part of the transmission line and being doped with a rare-earth element, and a pumping light source for supplying pumping light into the optical fiber for amplification. Here, optical isolators for inhibiting unnecessary light from propagating are disposed between the entrance end of the fiber optic amplifier and the entrance end of the optical fiber for amplification, and between the exit end of the fiber optic amplifier and the exit end of the optical fiber for amplification, respectively. Also, between these optical isolators, an optical coupler for guiding the pumping light emitted from the pumping light source into the optical fiber for amplification is disposed on at least one of the entrance end side and exit end side of the optical fiber for amplification.
In particular, the fiber optic amplifier according to the present invention is characterized in that it comprises a bleaching system for carrying out bleaching for the optical fiber for amplification. Here, bleaching refers to eliminating the defects generated in the glass constituting the optical fiber for amplification upon exposure to radiation, thereby restoring its original glass structure. This bleaching system reduces the transmission loss of signal light or pumping light caused by glass defects, whereby favorable signal amplification characteristics can be maintained.
Bleaching for the optical fiber for amplification includes optical bleaching and thermal bleaching, whereas the above-mentioned bleaching system carries out at least one of the optical bleaching and thermal bleaching. Here, it is not necessary for such bleaching to be carried out continuously while the fiber optic amplifier is operating, and similar effects can also be obtained when it is carried out at predetermined intervals.
In the case where the glass defects generated within the optical fiber for amplification are to be restored by optical bleaching, it is preferred that the bleaching system include a bleaching light source for supplying bleaching light having a wavelength shorter than that of the pumping light and a power higher than that of the pumping light into the optical fiber for amplification. In this case, between the above-mentioned optical isolators, an optical coupler for guiding the bleaching light from the bleaching light source into the optical fiber for amplification is disposed on one of the entrance end side and exit end side of the optical fiber for amplification.
Here, the wavelength of the above-mentioned bleaching light lies within the range of 0.2 xcexcm to 1.0 xcexcm, more preferably within the range of 0.2 xcexcm to 0.9 xcexcm. The bleaching light is not always required to be continuous light, and may be pulse light.
Further, for carrying out optical bleaching for selectively eliminating from the transmission line the bleaching light supplied into the optical fiber for amplification, the bleaching system preferably comprises a bleaching-light-eliminating optical coupler which is disposed between the above-mentioned optical isolators so as to hold, in cooperation with the bleaching-light-introducing optical coupler, the optical fiber for amplification. Due to this configuration, the bleaching light is prevented from being emitted outside the fiber optic amplifier together with the signal light (thus reducing noise).
In the case where the glass defects generated within the optical fiber for amplification are to be restored by thermal bleaching, on the other hand, the above-mentioned bleaching system preferably includes at least a heater for heating the optical fiber for amplification. Also, in order for the surface temperature of the optical fiber for amplification, which is an object to be heated, to be maintained within a predetermined range, this bleaching system comprises a temperature sensor for detecting the surface temperature of the optical fiber for amplification and a controller for carrying out temperature control of the heater according to measurement information from the temperature sensor.
The surface temperature of a specific optical fiber for amplification lies within the range of 50xc2x0 C. to 200xc2x0 C., more preferably within the range of 100xc2x0 C. to 150xc2x0 C.
In the fiber optic amplifier according to the present invention, the bleaching system can be configured so as to be able to carry out one or both of the above-mentioned optical bleaching and thermal bleaching.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given byway of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given-by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.